Conventional mechanical face seals are externally pressurized, so that the pressure is applied to the outside of the seal rings and the rings are under compression. In particular for high pressure applications, a barrier fluid at a pressure in excess of the product fluid is applied to the outside of the seal rings, while the interior of the rings is exposed to the process fluid. In this manner any leakage across the sealing faces will be of the barrier fluid, which is at higher pressure, into the process fluid, so that pollution of the environment is avoided. Even with externally pressurized seals of this type, the seal rings must be capable of withstanding internal pressures in emergencies, for example if subjected to reverse pressurization upon failure of the barrier fluid pressure.
It has consequently been proposed the reinforce the seal rings on their external periphery, for example as disclosed in EP 1375984 the disclosure of which is incorporated herein with reference thereto, in order to increase the internal pressures which the seal rings are capable of withstanding. Such composite seals are typically capable of withstanding internal pressures up to 250 bar.
While externally pressurized seals of the type disclosed above are suitable for many applications, in some circumstances it is desirable to expose the outside of the seal rings to process fluid, while the barrier fluid of applied internally of the seal rings. For example, when the process fluid contains solids, with externally pressurized seals where the process fluid is on the inside of the seal rings, the solids will be centrifuged into contact with the seal rings and associated components, clogging the components and causing the seal to hang and allowing leakage across the seal. When the process fluid is outside the seal rings, the solids will be centrifuged away from the seal rings and associated components.
Composite seals of the type disclosed above will allow internal pressurization. However the use of such composite rings with internal pressurization does present problems due to thermal distortion of the seal rings. With rings of this type, as the temperature of the sealing faces increases, the thermal gradient across the seal ring will cause the ring to distort so that the sealing face rotates outwardly. As a result of thermal distortion of the opposed seal rings, the gap between the sealing faces will increase from inside to outside.
When the barrier fluid is on the outside of the seal, this is not a problem, as the opening of the gap will increase the hydrostatic opening force in the barrier fluid between the sealing faces, which will reduce generated heat, maintaining equilibrium. In this manner, with external pressurization the effect of thermal distortion is inherently stabilized. However with internal pressurization, when the barrier fluid is on the inside of the seal rings, narrowing of the gap on the inside of the seal will reduce hydrostatic support, thereby increasing friction between the faces and generating a hot spot. Lapping of the sealing faces so that they are rotated inwardly when the seal rings are cold, will produce unacceptably high leakage rates at start-up when the pressure differential between the barrier fluid and process fluid is likely to be at its greatest. Moreover, even if the faces are lapped in this manner, as the faces heat up, the gap will reduce on the inside, reducing leakage and increasing thermal distortion. As a consequence thermal distortion of the rings cannot be stabilized in conventional manner. While it is possible to design seal rings which do rotate inwardly as the temperature gradient increases, such seal rings would not be suitable for internal pressurization which requires a ring of large radial section in order to withstand internal pressurization, even when reinforced externally.
The present invention provides an internally pressurized seal in which thermal distortion is stabilized.
According to one aspect of the present invention, an internally pressurized seal assembly comprises a first seal ring mounted in fixed axial and rotational relationship and sealed with respect to one of a pair of relatively rotatable components and a second seal ring moveable axially but fixed rotationally and sealed with respect to the other of the pair of relatively rotatably components, the second seal ring being urged resiliently towards the first seal ring, so that a radial sealing face of the first seal ring engages a radial sealing face of the second seal ring, a process chamber being formed at the inboard side of the seal rings, said process chamber opening to the outside of the seal rings, and a barrier chamber being provided at the outboard side of the seal rings, the barrier chamber opening to the inside of the seal rings, the external circumferential surface of the seal rings being shielded from process fluid in the process chamber by shroud members which ensure that heat transfer from the process fluid to the seal rings predominantly occurs at the outer regions of the seal rings adjacent the sealing faces, and a sleeve being secured internally of each seal ring which ensure that heat transfer between the seal rings and a barrier fluid in the barrier chamber predominantly occurs at the radially extending surfaces of the seal rings adjacent the sealing faces.
By concentrating heat transfer to and from the seal rings to the portions of the seal rings adjacent the sealing faces, in the manner described above, the temperature gradients in the seal rings, which results in rotation of the sealing faces due the thermal distortion, are significantly reduced, thereby reducing rotation of the sealing faces and minimizing the reduction in hydrostatic support.
According to a further embodiment of the invention hydrodynamic features are provided in the sealing face of one of the seal rings, these hydrodynamic features being in the form of grooves or recesses, which open to the internal periphery of the sealing faces. If heat generation at the sealing faces is increased the sealing faces rotate such that the gap between the sealing faces will increase inside to outside, this brings the hydrodynamic features into closer proximity to the other sealing face, increasing hydrodynamic support and reducing generated heat. In this manner with internal pressurization the effect of thermal distortion is further stabilized.
According to a further aspect of the present invention, a method of thermally stabilizing an internally pressurized seal having a first seal ring mounted in fixed axial and rotational relationship and sealed with respect to one of a pair of relatively rotatable components and a second seal ring moveable axially but fixed rotationally and sealed with respect to the other of the pair of relatively rotatably components, the second seal ring being urged resiliently towards the first seal ring so that a radial sealing face of the first seal ring engages a radial sealing face of the second seal ring, a process chamber being formed at the inboard side of the seal rings, said process chamber opening to the outside of the seal rings, and a barrier chamber being provided at the outboard side of the seal rings, the barrier chamber opening to the inside of the seal rings, the method comprises ensuring that heat transfer from a process fluid in the process chamber to the seal rings predominantly occurs at the outer regions of the seal rings adjacent the sealing faces and ensuring that heat transfer between the seal rings and a barrier fluid in the barrier chamber predominantly occurs at the radially extending faces of the seal rings adjacent the sealing faces.